1. Field of the Invention
The invention is generally related to the installation of piles in offshore structures and more particularly to tools used for handling piles during lowering and stabbing the piles underwater.
2. General Background
Piles are used in offshore marine structures to provide a foundation for support of structures such as jackets, towers, subsea templates, and tension leg platforms. Handling and installation of piles in offshore structures utilize various tools and appurtenances for lifting the piles from transportation vessels and positioning the pile properly in the corresponding leg or sleeve. Pile handling has evolved from utilizing simple padeyes welded to each pile section to use of various devices for handling and installing piles. Pile diameters and lengths used for marine structures have increased over the years, and water depths in which piles are installed now range to three thousand feet. In deep water, pile installation usually takes place below water. In order to reduce offshore handling time and allow for remote release of piles, several types of handling tools have been developed.
One type of pile handling tool in use is a radial gripping handling tool or internal elevator. These tools use radial gripping action to hold the pile or other pipe sections. The tool is hydraulically operated and utilizes gripping segments or slips which are forced outward to engage the pile. The internal elevator can be designed to handle a range of pile diameters and pile wall thicknesses. The internal elevator requires a hydraulic line, umbilical, which is connected to a power pack on the support vessel to engage or release the slips. Other options for engaging or releasing the slips include use of a hydraulic accumulator or Remotely Operated Vehicle (ROV) hot stab mounted on the internal elevator in lieu of using an umbilical. In deeper water depths, greater than one thousand feet, history has shown that the hydraulic umbilical is susceptible to handling problems and possible breakage. Other hydraulic release mechanisms such as the ROV hot stab or hydraulic accumulator require the ROV, using its manipulator, to actuate the elevators's hydraulic system.
Another pile handling tool utilizes an interlocking lug and cam arrangement to hold the pile (quick disconnect tool). This tool is designed to fit inside the pile with plate "cams" arranged such that when the tool is lowered inside the pile, these cams mate with corresponding lugs protruding from the pile's inside walls. Two lugs are typically used in this system. The cams are designed with slots that cause the tool to rotate as it is lowered onto the pile lugs. The pile lugs engage the cams as the tool is raised and supports the pile weight. After the pile is positioned and stabbed in the designated location, the pile is lowered until it "self supports". When the tool no longer supports the pile weight, it is lowered several feet farther which causes the tool to rotate, given the cam design, allowing the lugs to slide free and thus releasing the tool. This tool usually requires special machining of the pile to achieve the proper tolerance between the pile inside diameter and the cams. Therefore, the tool must be designed and fitted to a specific pile diameter and wall thickness.